The present invention relates to self-piercing rivets used to join together metallic sheets, and more particularly to a self-piercing rivet having spiraling which resists pull-out of the rivet with respect to metallic sheets joined thereby.
The joining of metallic sheets can be accomplished by various mechanical modalities, as for example, threaded fasteners, rivets and welding. The modality used depends upon the application. One type of rivet having an excellent ability to join together a stack of metallic sheets is a self-piercing rivet.
Various aspects of a prior art self-piercing rivet are shown at FIGS. 1 through 4. A self-piercing rivet 10 has a hollow shell 12 of cylindrical shape which is closed at one end by an overhanging head 14 and which has at the opposite end a point 16. A ram 18 and opposing die 20 are used to drive the self-piercing rivet 10, point 16 first, into two or more sheets 22, typically a metal, such as for example aluminum. The ram 18 has a convex contour 24 and the die has a concave contour 26, such that after stroking of the ram, the hollow shell 12 pierces the sheets 22 with a deformation D defined by the concave contour 26 and wherein the head 14 is countersunk by the convex contour 24. The deformation D involves a bending of the hollow shell 12 outwardly so as to lock the self-piercing rivet 10 in position with respect to the sheets 22 such as to prevent its removal from the sheets and, as a result, any possible disjoinder of the sheets.
It will be noticed that the deformation D prevents the self-piercing rivet 10 and the sheets 22 from releasing from one another under normal loading conditions. However, there is yet the possibility that under certain loading conditions the self-piercing rivet could be loosened from the sheets, in which case the sheets may then become mutually spaced apart or even separated from each other.
Accordingly, what is needed in the art is some way to prevent loosening of sheets mutually attached by a self-piercing rivet.
The present invention is a self-piercing rivet wherein the hollow shell thereof has an external spiral feature, wherein the spirals thereof constitute external surface features (intrusive or protrusive) which twist (or turn) about the axis of rotation of the hollow shell (or simply put, the shell axis) from generally between the head and the point. Since forces which will tend to loosen the rivet and separate the sheets must have a component parallel to the shell axis of the rivet, once the spiraled self-piercing rivet has been installed in a stack of sheets, the rivet is prevented from being loosened from the sheets because the spirals of the spiral feature resist interferingly (with respect to the sheets) any component of force which is parallel to the shell axis.
A preferred external spiral feature is provided by the exterior surface of the hollow shell having a cornered geometry (for example, a square, a hexagon, an octagon, etc.), wherein the corners are twisted about the shell axis from the head to the point. The pitch of the twist is preferably coarse; for example, between about one-quarter turn to about one or two turns over the length of the hollow shell.
In another preferred external spiral feature, irregular surface features extend twistingly generally between the head and the point. The irregular surface features may be either protruding surface features, such as for example ribs, or intruding surface strutures, such as for example flutes (i.e., grooves or slots).
In operation, as the spiraled self-piercing rivet is driven into a plurality of sheets (composed of for example, metal or metal composite) to be joined, the spiral features interact with the sheets such that after joinder, the sheets are prevented from mutual separation because the component of applied forces which is parallel to the shell axis is resisted by an interference relationship between the spiral features and the stack of sheets.
Accordingly, it is an object of the present invention to provide a self-piercing rivet having a spiral feature which serves to resist loosening of the rivet with respect to the sheets joined thereby.
This and additional objects, features and advantages of the present invention will become clearer from the following specification of a preferred embodiment.